Add input validation guidelines and fix the case where there are null defaults or tri-state booleans

README.md

@@ -458,6 +458,17 @@ npx @modelcontextprotocol/inspector --cli https://my-mcp-server.example.com --me
 | **Automation**           | N/A                                                                       | Ideal for CI/CD pipelines, batch processing, and integration with coding assistants                                                                  |
 | **Learning MCP**         | Rich visual interface helps new users understand server capabilities      | Simplified commands for focused learning of specific endpoints                                                                                       |
 
+## Tool Input Validation Guidelines
+
+When implementing or modifying tool input parameter handling in the Inspector:
+
+- **Omit optional fields with empty values** - When processing form inputs, omit empty strings or null values for optional parameters, UNLESS the field has an explicit default value in the schema that matches the current value
+- **Preserve explicit default values** - If a field schema contains an explicit default (e.g., `default: null`), and the current value matches that default, include it in the request. This is a meaningful value the tool expects
+- **Always include required fields** - Preserve required field values even when empty, allowing the MCP server to validate and return appropriate error messages
+- **Defer deep validation to the server** - Implement basic field presence checking in the Inspector client, but rely on the MCP server for parameter validation according to its schema
+
+These guidelines maintain clean parameter passing and proper separation of concerns between the Inspector client and MCP servers.
+
 ## License
 
 This project is licensed under the MIT License—see the [LICENSE](LICENSE) file for details.

client/src/components/ToolsTab.tsx

@@ -179,7 +179,18 @@ const ToolsTab = ({
                                 onCheckedChange={(checked: boolean) =>
                                   setParams({
                                     ...params,
-                                    [key]: checked ? null : prop.default,
+                                    [key]: checked
+                                      ? null
+                                      : prop.default !== null
+                                        ? prop.default
+                                        : prop.type === "boolean"
+                                          ? false
+                                          : prop.type === "string"
+                                            ? ""
+                                            : prop.type === "number" ||
+                                                prop.type === "integer"
+                                              ? undefined
+                                              : undefined,
                                   })
                                 }
                               />

client/src/components/__tests__/ToolsTab.test.tsx

@@ -177,6 +177,103 @@ describe("ToolsTab", () => {
     });
   });
 
+  it("should support tri-state nullable boolean (null -> false -> true -> null)", async () => {
+    const mockCallTool = jest.fn();
+    const toolWithNullableBoolean: Tool = {
+      name: "testTool",
+      description: "Tool with nullable boolean",
+      inputSchema: {
+        type: "object" as const,
+        properties: {
+          optionalBoolean: {
+            type: ["boolean", "null"] as const,
+            default: null,
+          },
+        },
+      },
+    };
+
+    renderToolsTab({
+      tools: [toolWithNullableBoolean],
+      selectedTool: toolWithNullableBoolean,
+      callTool: mockCallTool,
+    });
+
+    const nullCheckbox = screen.getByRole("checkbox", { name: /null/i });
+    const runButton = screen.getByRole("button", { name: /run tool/i });
+
+    // State 1: Initial state should be null (input disabled)
+    const wrapper = screen.getByRole("toolinputwrapper");
+    expect(wrapper.classList).toContain("pointer-events-none");
+    expect(wrapper.classList).toContain("opacity-50");
+
+    // Verify tool is called with null initially
+    await act(async () => {
+      fireEvent.click(runButton);
+    });
+    expect(mockCallTool).toHaveBeenCalledWith(toolWithNullableBoolean.name, {
+      optionalBoolean: null,
+    });
+
+    // State 2: Uncheck null checkbox -> should set value to false and enable input
+    await act(async () => {
+      fireEvent.click(nullCheckbox);
+    });
+    expect(wrapper.classList).not.toContain("pointer-events-none");
+
+    // Clear previous calls to make assertions clearer
+    mockCallTool.mockClear();
+
+    // Verify tool can be called with false
+    await act(async () => {
+      fireEvent.click(runButton);
+    });
+    expect(mockCallTool).toHaveBeenLastCalledWith(
+      toolWithNullableBoolean.name,
+      {
+        optionalBoolean: false,
+      },
+    );
+
+    // State 3: Check boolean checkbox -> should set value to true
+    // Find the boolean checkbox within the input wrapper (to avoid ID conflict with null checkbox)
+    const booleanCheckbox = within(wrapper).getByRole("checkbox");
+
+    mockCallTool.mockClear();
+
+    await act(async () => {
+      fireEvent.click(booleanCheckbox);
+    });
+
+    // Verify tool can be called with true
+    await act(async () => {
+      fireEvent.click(runButton);
+    });
+    expect(mockCallTool).toHaveBeenLastCalledWith(
+      toolWithNullableBoolean.name,
+      {
+        optionalBoolean: true,
+      },
+    );
+
+    // State 4: Check null checkbox again -> should set value back to null and disable input
+    await act(async () => {
+      fireEvent.click(nullCheckbox);
+    });
+    expect(wrapper.classList).toContain("pointer-events-none");
+
+    // Verify tool can be called with null again
+    await act(async () => {
+      fireEvent.click(runButton);
+    });
+    expect(mockCallTool).toHaveBeenLastCalledWith(
+      toolWithNullableBoolean.name,
+      {
+        optionalBoolean: null,
+      },
+    );
+  });
+
   it("should disable button and change text while tool is running", async () => {
     // Create a promise that we can resolve later
     let resolvePromise: ((value: unknown) => void) | undefined;

client/src/utils/__tests__/paramUtils.test.ts

@@ -205,4 +205,112 @@ describe("cleanParams", () => {
       // optionalField omitted entirely
     });
   });
+
+  it("should preserve null values when field has default: null", () => {
+    const schema: JsonSchemaType = {
+      type: "object",
+      required: [],
+      properties: {
+        optionalFieldWithNullDefault: { type: "string", default: null },
+        optionalFieldWithoutDefault: { type: "string" },
+      },
+    };
+
+    const params = {
+      optionalFieldWithNullDefault: null,
+      optionalFieldWithoutDefault: null,
+    };
+
+    const cleaned = cleanParams(params, schema);
+
+    expect(cleaned).toEqual({
+      optionalFieldWithNullDefault: null, // preserved because default: null
+      // optionalFieldWithoutDefault omitted
+    });
+  });
+
+  it("should preserve default values that match current value", () => {
+    const schema: JsonSchemaType = {
+      type: "object",
+      required: [],
+      properties: {
+        fieldWithDefaultString: { type: "string", default: "defaultValue" },
+        fieldWithDefaultNumber: { type: "number", default: 42 },
+        fieldWithDefaultNull: { type: "string", default: null },
+        fieldWithDefaultBoolean: { type: "boolean", default: false },
+      },
+    };
+
+    const params = {
+      fieldWithDefaultString: "defaultValue",
+      fieldWithDefaultNumber: 42,
+      fieldWithDefaultNull: null,
+      fieldWithDefaultBoolean: false,
+    };
+
+    const cleaned = cleanParams(params, schema);
+
+    expect(cleaned).toEqual({
+      fieldWithDefaultString: "defaultValue",
+      fieldWithDefaultNumber: 42,
+      fieldWithDefaultNull: null,
+      fieldWithDefaultBoolean: false,
+    });
+  });
+
+  it("should omit values that do not match their default", () => {
+    const schema: JsonSchemaType = {
+      type: "object",
+      required: [],
+      properties: {
+        fieldWithDefault: { type: "string", default: "defaultValue" },
+      },
+    };
+
+    const params = {
+      fieldWithDefault: null, // doesn't match default
+    };
+
+    const cleaned = cleanParams(params, schema);
+
+    expect(cleaned).toEqual({
+      // fieldWithDefault omitted because value (null) doesn't match default ("defaultValue")
+    });
+  });
+
+  it("should fix regression from issue #846 - tools with multiple null defaults", () => {
+    // Reproduces the exact scenario from https://github.com/modelcontextprotocol/inspector/issues/846
+    // In v0.17.0, the cleanParams function would remove all null values,
+    // breaking tools that have parameters with explicit default: null
+    const schema: JsonSchemaType = {
+      type: "object",
+      required: ["requiredString"],
+      properties: {
+        optionalString: { type: ["string", "null"], default: null },
+        optionalNumber: { type: ["number", "null"], default: null },
+        optionalBoolean: { type: ["boolean", "null"], default: null },
+        requiredString: { type: "string" },
+      },
+    };
+
+    // When a user opens the tool in Inspector, fields initialize with their defaults
+    const params = {
+      optionalString: null, // initialized to default
+      optionalNumber: null, // initialized to default
+      optionalBoolean: null, // initialized to default
+      requiredString: "test",
+    };
+
+    const cleaned = cleanParams(params, schema);
+
+    // In v0.16, null defaults were preserved (working behavior)
+    // In v0.17.0, they were removed (regression)
+    // This fix restores the v0.16 behavior
+    expect(cleaned).toEqual({
+      optionalString: null,
+      optionalNumber: null,
+      optionalBoolean: null,
+      requiredString: "test",
+    });
+  });
 });

client/src/utils/paramUtils.ts

@@ -2,7 +2,7 @@ import type { JsonSchemaType } from "./jsonUtils";
 
 /**
  * Cleans parameters by removing undefined, null, and empty string values for optional fields
- * while preserving all values for required fields.
+ * while preserving all values for required fields and fields with explicit default values.
  *
  * @param params - The parameters object to clean
  * @param schema - The JSON schema defining which fields are required
@@ -14,13 +14,23 @@ export function cleanParams(
 ): Record<string, unknown> {
   const cleaned: Record<string, unknown> = {};
   const required = schema.required || [];
+  const properties = schema.properties || {};
 
   for (const [key, value] of Object.entries(params)) {
     const isFieldRequired = required.includes(key);
+    const fieldSchema = properties[key] as JsonSchemaType | undefined;
+
+    // Check if the field has an explicit default value
+    const hasDefault = fieldSchema && "default" in fieldSchema;
+    const defaultValue = hasDefault ? fieldSchema.default : undefined;
 
     if (isFieldRequired) {
       // Required fields: always include, even if empty string or falsy
       cleaned[key] = value;
+    } else if (hasDefault && value === defaultValue) {
+      // Field has a default value and current value matches it - preserve it
+      // This is important for cases like default: null
+      cleaned[key] = value;
     } else {
       // Optional fields: only include if they have meaningful values
       if (value !== undefined && value !== "" && value !== null) {